In recent years, further development of commercial value of products has been required with the spread of EPS. In such development, there is a requirement for keeping an assist function in case of a failure even though the assist function has a time limit for operation. For such assistance, there has been proposed a method in which in place of using an abnormal output signal from a sensor, an output signal from the other sensor is practically used as a back-up signal, which is disclosed in Patent Document 1.